1. Field of the Invention
The present invention relates to a glass for a substrate which is used in a flat display substrate such as a liquid crystal display and an EL display.
2. Description of the Related Art
An alkali metal-free (referred to hereinafter simply as “alkali-free”) glass substrate has been widely used as a flat display substrate such as a liquid crystal display and an EL display.
In particular, since an electron device such as a thin film transistor-type active matrix liquid crystal display (TFT-LCD) is a thin-type and has a small consumption power, the device has been used in various utilities such as car navigation and a finder of a digital camera and, in recent years, a monitor of a personal computer and a TV.
In a TFT-LCD panel maker, improvement in productivity and cost reduction are tried by manufacturing plural devices on a glass substrate, which formed by a glass maker, and dividing into every device to obtain a product. In recent years, in utilities such as a monitor of a personal computer or a TV, a large size has been required in a device itself and, in order to produce a plurality of such kind of large devices at one time, therefore a glass substrate having a large area such as 1000×1200 mm has been required.
In addition, in a portable-type device such as a cellular phone and a note-type personal computer, for convenience for carrying, weight reduction of an instrument is required, and weight reduction is also required in a glass substrate. In order to reduce a weight of a glass substrate, thickness reduction of a substrate is effective and, currently, a standard thickness of a glass substrate for TFT-LCD is very thin such as about 0.7 mm.
However, the aforementioned large-sized thin glass substrate has great sag due to a self-weight, and this has become a great problem in a manufacturing step.
That is, this kind of a glass substrate is formed by a glass maker, and passed through steps such as cutting, annealing, inspection and washing. During these steps, the glass substrate is placed into or taken out from a cassette in which a multiple-staged shelf is formed. This cassette can hold glass substrates in a horizontal direction such that both sides or three sides of the glass substrate are supported only by shelves formed on left and right inside walls, or left, right and back inside walls. However, since a large-sized thin glass substrate has a large amount of sag, when a glass substrate is placed into a cassette, a part of the glass substrate is contacted with the cassette or another glass substrate leading to breakage, or when the glass substrate is taken out from the cassette, it is greatly rocked, easily leading to unstability. On the other hand, in a display maker, since the same form of the cassette is used, the similar problem arises.
A sag amount due to a self-weight of such the glass substrate varies in proportion to a glass density and in inverse proportion to a Young's modulus. Accordingly, in order to suppress a sag amount of a glass substrate small, it is necessary that a specific Young's modulus expressed by a ratio of Young's modulus/density is increased. In order to increase a specific Young's modulus, a glass material having a high Young's modulus and a low density becomes necessary and, at the same specific Young's modulus, in a glass having a lower density, a thickness of a glass sheet having the same weight can be increased by a portion of a reduced weight. Since a sag amount of a glass varies in reverse proportion to a square of a sheet thickness, effect of reducing sag derived from increase in a sheet thickness is very great. Since reduction in a glass density has great effect also on weight reduction of a glass, the glass having as smaller as possible density is better.
Generally, this kind of an alkali-free glass contains a relatively large amount of an alkaline earth metal oxide. In order to reduce a density of the glass, it is effective to reduce a content of the alkaline earth metal oxide. However, since the alkaline earth metal oxide is a component which promotes meltability of a glass, meltability is reduced when the content is decreased. When meltability of a glass is reduced, internal defects such as seeds and stones easily occur in the glass. Since seeds and stones in a glass prevent transmission of light, this becomes a fatal defect to the glass substrate for display. In order to suppress such the internal defects, a glass must be melted at a high temperature for a long period of time. On the other hand, melting at a high temperature increases a burden on a glass melting furnace. A refractory used in the furnace is more eroded at a higher temperature, and a life cycle of the furnace becomes shorter.
In addition, in this kind of a glass substrate, thermal shock resistance is also an important requirement. A fine flaw and crack are present on an end face of a glass substrate even when chamfered and, when a tensile stress due to heat is exerted by concentrating on the flaw and the crack, the glass substrate is broken in some cases. Breakage of a glass substrate not only decreases an operation rate, but also a fine glass powder produced upon breakage is adhered on other glass substrates, and this causes disconnection and deteriorated patterning, thus, there is a possibility that manufacturing circumstances are contaminated.
Meanwhile, as a recent direction of development of TFT-LCD, in addition to increase in a size of a screen and weight reduction, increase in performance such as a higher definition, a higher speed response and a higher aperture ratio is exemplified. In particular, in recent years, for the purpose of increase in performance and weight reduction of a liquid crystal display, polycrystalline silicon TFT-LCD (p-Si.TFT-LCD) has been extensively developed. In the previous p-Si.TFT-LCD, since a temperature at its manufacturing step is very high such as 800° C. or higher, only a quartz glass substrate could be used. However, by recent development, a temperature at a manufacturing step is lowered to 400 to 600° C., and an alkali-free glass substrate has become to be used as in amorphous silicon TFT-LCD (a-Si.TFT-LCD) which is currently produced at a large scale.
In a step of manufacturing p-Si.TFT-LCD, since there is many heat-treating steps and a glass substrate is repeatedly heated rapidly and cooled rapidly as compared with a step of manufacturing a-Si.TFT-LCD, thermal shock on the glass substrate is further increased. Further, a size of a glass substrate is increased as described above, not only there easily arises a difference in a temperature of the glass substrate, but also a possibility that fine flaws and cracks are produced on an end face is also increased. Therefore a possibility that the substrate is broken during a heating step is increased. The most fundamental and effective method for solving this problem is to reduce a thermal stress generated from a difference in thermal expansion and, for this reason, a glass having a low thermal expansion coefficient is required. In addition, since when a difference in thermal expansion between a thin film transistor (TFT) material and glass becomes great, warpage occurs in the glass substrate, it is also required to have a thermal expansion coefficient approximate to that (about 30 to 33×10−7° C.) of a TFT material such as p-Si.
In addition, it is said that a temperature at a step of manufacturing p-Si.TFT-LCD has been lowered recently, but the temperature is still significantly higher as compared with a temperature at a step of manufacturing a-Si.TFT-LCD. If a glass substrate has a low heat resistance, when the glass substrate is exposed to a high temperature of 400 to 600° C. during the step of manufacturing p-Si.TFT-LCD, fine dimensional shrinkage called thermal compaction is caused, and this causes variance of a pixel pitch of TFT, and this may be a cause for deteriorated display. In addition, if a glass substrate has a further lower heat resistance, there is a possibility that deformation and warpage of the glass substrate are caused. Further, also in order to prevent occurrence of pattern shift by thermal compaction of a glass substrate at a step of manufacturing a LCD such as film deposition step, the glass excellent in heat resistance is required.
Further, on a surface of a glass substrate for TFT-LCD, a transparent electrically conductive film, an insulating film, a semiconductor film and a metal film are formed and, moreover, various circuits and patterns are formed by photolithography etching (photoetching). In addition, in these film formation and photoetching step, the glass substrate is subjected to various heat treatment and chemical treatment.
Therefore, when an alkali metal oxide (Na2O, K2O, Li2O) is contained in a glass, it is thought that an alkali metal (referred to hereinafter simply as “alkali”) ion is diffused into a formed semiconductor substance film during heat treatment, leading to deterioration of film property, and it is required that an alkali metal oxide is not substantially contained. Further, it is required that such the chemical resistance is possessed that deterioration is not caused by chemicals such as various acids and alkalis used in a photoetching step.
In addition, a glass substrate for TFT-LCD is formed mainly by a down-draw process or a float process. Examples of a down-draw process include a slot down-draw process and an overflow down-draw process and, since a glass substrate formed by the down-draw process does not need polishing process, there is an advantage that cost reduction is easy. However, when a glass substrate is formed by a down-draw process, since the glass is easily devitrified, the glass excellent in devitrification resistance is required.
Then, an alkali-free glass for a substrate characterized in that the aforementioned various properties are satisfied and, in particular, a low density, low expansion, and a high strain point are possessed is proposed (e.g. Japan Unexamined Patent Publication JP-A No. 2002-308643).
The alkali-free glass having a low density, low expansion and a high strain point disclosed in JP-A No. 2002-308643 has a density of 2.45 g/cm3 or a lower, an average thermal expansion coefficient in a temperature range of 30 to 380° C. of 25 to 36×10−7/° C., and a strain point of 640° C. or higher, thus, the aforementioned requirements are satisfied. However, the aforementioned alkali-free glass has a melting temperature (temperature corresponding to 102.5 poise) of approximately 1580° C. or higher, and high temperature melting is necessary.
Then, electric melting is frequently applied to high temperature melting of such the glass. In the case of electric melting, a glass melting furnace is usually constructed of an alumina electrocast refractory having a high electric resistance. However, the alumina electrocast refractory is easily eroded by glass melt as compared with, for example, a high zirconia refractory, and has a short life. In particular, when an alumina electrocast refractory is used in a furnace for melting a glass requiring the aforementioned high temperature melting, the refractory is eroded in a short period of time, and stable operation can not be performed over a long period of time. As a result, a melting furnace must be frequently repaired, productivity is reduced, and a facility cost is increased. In addition, when an alumina electrocast is used, lots of seeds are generated from the refractory.
Under such the circumstances, use of an electric melting furnace using a high zirconia refractory having erosion resistance and hardly generating seeds is being studied. However, when the aforementioned glass having a low density, low expansion, and a high strain point is electrically melted in a melting furnace using a high zirconia refractory, there arises a problem that the glass is easily devitrified in a later forming step.